This work uses high precision isotope ratio mass spectrometry to determine the abundance of 13CO2 in respiratory CO2 after the administration of 13C labeled compounds to neonatal, pediatric, obstetric and lactating patients. In the coming year we will determine the excretion of labeled CO2 following intravenous bicarbonate administration to establish the variation and contribution of bicarbonate pools to the end products analysis of substrate oxidation rates. These studies will lay the ground work for measurements of essential amino acid oxidation by infants receiving total parenteral nutrition in longitudinal studies of protein nutritional status in these subjects. Intestinal maturation of infants receiving formula feedings compared to human milk fortified with human milk products will be assessed by lipid absorption studies with labeled triglycerides of medium and long chain fatty acids.